1. Field of the Invention
The present invention relates to a ferroelectric register, and more particularly to a ferroelectric register configured to reduce probability of data storage failure by connecting a plurality of ferroelectric capacitors in parallel, thereby improving data storage reliability and stability.
2. Description of the Background Art
In general, a ferroelectric random access memory (FeRAM) has a data processing speed equivalent to a dynamic random access memory (DRAM), and preserves data even when power is off.
The FeRAM is a memory having a similar structure to the DRAM. The FeRAM employs a ferroelectric substance to form capacitors, and thus uses high remanent polarization which is a property of the ferroelectric substance. Even if electric fields are removed, data are not deleted in the FeRAM due to the remanent polarization.
FIG. 1 is a hysteresis loop provided to explain properties of a general ferroelectric capacitor.
Referring to FIG. 1, although electric fields are removed in electric field induced polarizations, the ferroelectric capacitor maintains a predetermined amount (A and D) of remanent polarizations (or spontaneous polarizations).
An FeRAM cell introduces the states (D and A) of the remanent polarization to data ‘1’ and ‘0’, respectively, and applies them to memory elements.
FIG. 2 is a diagram illustrating a unit cell of the general FeRAM.
As illustrated in FIG. 2, a bit line BL is formed in one direction, and a word line WL is formed in the direction crossing the bit line BL. A plate line PL is formed in parallel to the word line WL at a predetermined interval. An NMOS transistor has its gate terminal connected to the word line WL, and its source terminal connected to the bit line BL. A ferroelectric capacitor FC has its first terminal connected to the drain terminal of the NMOS transistor, and its second terminal connected to the plate line PL.
In the normal state, the ferroelectric capacitor FC has hysteresis properties of loop A of FIG. 3, but in the weak state, the ferroelectric capacitor FC has deformed properties of loop B of FIG. 3. Therefore, the weak state remanent polarization is much smaller than the normal state remanent polarization.
If the weak state ferroelectric capacitor is used, the remanent polarization thereof is so small that a register may not store nonvolatile data in a power-up mode.